1. Field of the Invention
The present invention relates in general to a thin-film condenser formed on the surface of an insulation substrate and more specifically to a thin-film condenser having a lower electrode layer with a view to improving the heat-resisting property of the condenser.
2. Description of the Prior Art
By recent development in the integrated circuit technology, the operating frequency of the integrated circuit has been elevated remarkably for the reason of a high-speed operation thereof. This is attended with, in order to prevent an erroneous operation of the integrated circuit due to noise, a practice of installing or forming a decoupling condenser on a ceramic substrate or the like insulation substrate.
It has been considered, notwithstanding the condenser is required to have a relatively large electrostatic capacity, to form the condenser on the insulation substrate adjacent the integrated circuit for the reason of the insulation substrate of itself being required to be compact and for the reason of the properties of the condenser.
In this instance, in order that a larger electrostatic capacity can be attained in a limited area, it is required to make thinner the thickness of the dielectric layer, so that a thin-film dielectric layer is formed by sputtering, CVD (chemical vapor deposition) method, sol-gel method, LB(Langmuir-Blodgett) film method or the like thin-film forming method.
Of those methods, well known as disclosed in Japanese patent application laid-open publication 52-53257, Japanese patent application after examination publication 60-55975, etc. is a method comprising the steps of forming a thin-film from Ta (tantalum) or the like metal that can carry out anodic oxidation, by sputtering or the like on an insulation substrate, and thereafter making the thin-film carry out anodic oxidation by using citric acid solution of the density of 0.1% for instance, for thereby forming a lower electrode made of Ta and a dielectric layer made of Ta.sub.2 O.sub.5. The method further comprises the step of forming on the dielectric layer an upper electrode of Ta, Al or the like metal by sputtering or the like method, for thereby forming a thin-film condenser.
Thereafter, the insulation substrate is subjected to a heat treatment at about 300.degree..about.500.degree. C. for forming an insulation layer by curing of polyimide or the like, or for installation of a semiconductor chip (die attach), or for closure by a lid or cover. Accordingly, a thin-film condenser formed on an insulation substrate is also required to have a heat-resisting property.
However, in case of the thin-film condenser formed by the above described prior art method, Ta in the lower electrode is diffused into the dielectric layer made of Ta.sub.2 O.sub.5 by heat, thus causing the dielectric layer to exhibit a conductive property.
Since the thin-film dielectric layer is thin, the insulation property of the thin-film dielectric layer of itself is lowered by such transformation, so the condenser cannot fully exhibit a required property.
Accordingly, it has been impossible to attain a heat-resisting property above 300.degree. C.
In view of such a problem, with a view to preventing diffusion of Ta, etc., a search was conducted by the applicant on the heat-resisting property of the condenser by forming the upper and lower electric layers from Cu(copper), Ag(silver), or the like metal that does not cause diffusion with Ta.sub.2 O.sub.5 and forming the thin-film dielectric layer from Ta.sub.2 O.sub.5.
However, such a condenser is conversely lowered in the insulation property to cause a problem of a short circuit defect, resulting in that it was impossible to improve the heat-resisting property of the condenser.
This is because the crystals of Cu (copper), etc. in the electrode layers grew into grains at the temperature of 200.degree..about.250.degree. C. so than movement of the Cu atoms, etc. was caused to damage the thin-film dielectric layer in contact with those atoms. Further, Cu and Ag are larger in coefficient of thermal expansion as compared with the dielectric layer, so it is considered that it was due to an influence of thermal stress that was caused between the thin-film dielectric layer and each electrode layer.
Furthermore, the damage of the thin-film dielectric layer was caused more pronouncedly by the grain growth and thermal stress of the lower electrode layer.
Because the upper electrode layer was constituted by a plurality of smaller electrode sections with a view to making smaller the influence of the short circuit defect of the condenser caused by the defect of the dielectric layer.
That is, the small electrode sections constituting the upper electrode layer, are smaller in the size of electrode as compared with the lower electrode layer so that the lower electrode layer becomes larger in the influence of grain growth and thermal stress on the dielectric layer.